Writers very rarely reread their own work. Why would they? Anne Enright recently wrote, in this newspaper, that "a book is only properly done when you can't look at it without throwing up". It takes so long to write a book, and requires so many revisions, that by the end of it your book seems as appealing as your best friend might if you'd been stuck in solitary confinement with him for two years.

The nausea eventually recedes. By the time the book comes out, assuming critics and readers don't despise the finished product, it is hard not to feel a certain degree of pride. The day your first copy arrives in the post you look at it close up, you look at it from far away, you reread the blurb, you probably stroke and fondle it for a while; but do you turn to page one and start reading? Oh, no. No, no, no. The book is done. Over. If you are still thinking about those characters, you are in a psychological zone comparable to that of middle-aged men who still daydream about the goals they scored for their school football team.

So when your novel is adapted for screen or stage, it is impossible not to be curious – and also a little afraid. My first novel, New Boy, was recently turned into a play, and as I made my way to the theatre on opening night, I felt I was attending the birth of something new, as well as the exhumation of something embarrassingly old. I wrote the novel, which is about the homoerotically homophobic sexual angst of teenage boys in 1980s suburban London, roughly 15 years ago. Could something really be dug up after so long without any risk of putrefaction?

The stage show stars Nicholas Hoult, one of the charismatic leads in E4's cult hit Skins. He would have been a toddler when I was writing the book. I distinctly remember, during one of the first holidays I ever took with the girlfriend who became my wife, retreating from the Italian heat into an air-conditioned cinema and watching Nicholas Hoult, as a boy, in About a Boy. Now, almost a decade later, I find myself sitting in a West End theatre, watching him play a character I made up 15 years ago, based on a comic version of what I might conceivably have been like as a teenager, 10 years before that.

The novel was, like many first novels, largely a roman à clef. It is set in the suburb of north London where I grew up, and revolves around a dissection of the social mores, hierarchies and groupings of that world. The central relationship of the novel, between two boys struggling with their burgeoning sexuality, is almost entirely fictional - but the world in which it is set is as meticulously accurate as I could make it.

Watching Hoult speak long-forgotten lines, utterly familiar to me the moment they came out of his mouth, felt like a bizarre telescoping of multiple periods of my life. The present began to seem strangely commingled with my now impossible-to-remember teenage years. It is my name on the novel, but I am no longer that person, just as I was no longer the teenager I was portraying when I wrote the book in my 20s. Physicists talk about wormholes as shortcuts through space and time. Watching New Boy on stage was like finding myself in a leisure centre full of wormhole aquaslides.

In live theatre, when something happens, it really happens. An argument in the novel becomes two people standing in front of you, shouting, their voices cracking, saliva flying. A cruelly exaggerated comic portrait of my brother, which started life as a private joke, now came alive on stage; and, extraordinarily, it contained more than a glimmer of my real sibling.

The magic of adaptation is that when it is done well - as it has been here, by Russell Labey - this old thing really does become new again. As the audience around me laughed at the jokes, I realised I was the only person there for whom this was an old thing.

Some authors imagine that changing their material for a new medium can only do it harm. But sitting with a group of strangers, hearing them laughing at words written by me, albeit long ago, has proved an intense thrill. As a novelist, you hope you are entertaining your audience, but you never see or hear it. Watching New Boy on stage still didn't make me want to reread my novel, but it did make me want to get up the next day and carry on writing.